The first evidence for the importance of heavy chain phosphorylation was our data in the early 1980s showing that actin-activated ATPase activity of Acanthamoeba myosin II (AMII) was inhibited by phosphorylation of 1 to 3 serine residues in the 29-residue non-helical tailpiece of each of the two heavy chains. For a period of about 10 years from the mid-1980s to mid-1990s, we had investigated the possible mechanism of this inhibition. The one consistent result was that heteropolymers of unphosphorylated (active) myosin and phosphorylated (inactive) myosin had significantly less actin-activated ATPase activity than equivalent mixtures of homopolymers, i.e. regulation of AMII was at the level of filaments with phosphorylated AMII inactivating unphosphorylated AMII when both are in the same filaments. Interestingly, the phosphorylation sites at the C-terminal end of the tail are 80-nm distant from the ATPase site in the globular separated by a coiled-coil helix. At that time, neither we nor others had any evidence that phosphorylation of the non-helical tailpiece affected the filament structure. We are now reinvestigating this problem by studying the structure and actin-activated ATPase activity of expressed wild-type and mutated AMII.